1. Field of the Invention
This invention relates to a solid-state imaging device advantageously employed for a camera unit mainly used for industrial purposes for imaging an object moving at a fast speed. More particularly, it relates to a solid-state imaging device enabling a high-speed shutter operation.
2. Description of the Related Art
The present Applicant has proposed in U.S. Pat. No. 5,157,502 a solid-state imaging device having an electronic shutter function of performing light exposure adjustment without employing an iris mechanism by controlling the electric charge storage time of a field storage type solid-state imaging device (CCD image sensor).
With the above solid-state imaging device, the electrical charges stored in the CCD image sensor are read out by a high-level picture readout pulse, shown in FIG. 1(b), outputted during a vertical blanking period (VBLK) in which a vertical blanking signal shown in FIG. 1(a) goes low. The charge storage time in the CCD image sensor is controlled by a reset pulse shown in FIG. 1(c), such that the electrical charges stored therein are drained in an overflow drain when the reset pulse is supplied thereto. Consequently, no electrical charges are stored in the CCD image sensor during the charge draining period, that is during the time period when the reset pulses are supplied. Therefore, electrical charges are stored in the CCD image sensor since the time the reset pulses cease to be supplied to the CCD image sensor, and the charge storage time in the CCD image sensor, that is the shutter speed, may be controlled by controlling the timing of cessation of the reset pulses.
The solid-state imaging device is particularly suited to the capturing of an object moving at a fast speed because the shutter speed can be varied responsive to the movement of the object by employing such electronic shutter function.
There is known a solid-state imaging device which is employed mainly for industrial purposes for imaging a moving object.
Such solid-state imaging device is arranged as shown for example in FIG. 2 in which, when an object 2 travelling on a path of movement 1 is moved past an imaging section 3, the object 2 is detected by a position sensor 4 which then outputs a low-level trigger pulse shown at timing t11 in FIG. 3(a) to a shutter pulse generator 5.
When supplied with the low-level trigger pulse, the shutter pulse generator 5 supplies a low-level shutter pulse to the CCD control circuit 6. That is, when supplied with the low-level trigger pulse, the shutter pulse generator 5 outputs a low-level shutter pulse at timing t11 shown in FIG. 3(b) to the CCD control circuit 6.
As long as the low-level shutter pulse is supplied, the CCD control circuit 6 supplies the reset pulses for draining the charges stored in the CCD image sensor 7. In this manner, as long as the reset pulses are supplied, imaging by the CCD image sensor 7 does not take place. However, if simultaneously the low-level trigger pulse is supplied, the CD control circuit 6 ceases to supply the reset pulses to the CCD image sensor 7, after supplying of the last trigger pulse. This initiates storage of electrical charges in the CCD image sensor 7.
The CCD control circuit 6 is supplied with the low-level vertical synchronizing signal shown between timing t11 and timing t12 in FIG. 3(c), and a horizontal synchronizing signal shown in FIG. 3(c), from synchronizing signal generator 8. When supplied with the above-mentioned shutter pulse, the CCD control circuit 6 supplies a high-level readout pulse, shown at timing t13 in FIG. 3(e), to the CCD image sensor 7, after counting e.g. nine pulses of the horizontal synchronizing signal shown in FIG. 3(d), since the decay of the vertical synchronizing signal shown in FIG. 3(c), and subsequently counting hundreds of clock pulses. In this manner, electrical charges are stored in the CCD image sensor 7 in an amount correlated with the imaging light radiated via an imaging lens 9 since the timing t11 when the shutter pulse is supplied to the CCD image sensor 7 until the timing t13 shown in FIG. 3(e) when the readout pulse is supplied to the CCD image sensor 7. The time period of t11-t13 represents a shutter speed.
Meanwhile, readout of the electrical charges from the CCD image sensor V is performed during a vertical blanking period which is the time period since timing t11 until timing t14 shown in FIG. 3(f).
The electrical charges read out from CCD image sensor 7 are supplied as imaging signal to an imaging signal processor 10. The imaging signal processor 10 performs signal processing, such as appendage of synchronizing signals, on the imaging signals, before outputting the processed signal at an output terminal 11. The imaging signal outputted at the output terminal 11 is supplied to e.g. a display screen of an analyzer. This enables the status of the object 2 to be analyzed in case the latter has been moved.
Since the solid-state imaging device, which effects imaging of a moving object, is employed mainly for industrial purposes, there are occasions wherein it is desired that the object 2 shown in FIG. 2 be moved at a fast speed and imaged by the high-speed shutter operation of e.g. a 10,000th of a second.
However, with the above-described solid-state imaging device, in which the readout pulse is supplied to the CCD image sensor after counting e.g. nine horizontal synchronizing pulses since the decay of the vertical synchronizing pulse and subsequently counting hundreds of clock pulses, the output timing of the readout pulse is pre-set on the basis of pixel array of the CCD image sensor employed in the solid-state imaging device.
The purpose of such arrangement is to prevent optical black pixels (OPB), provided for producing a reference black level at the lower end of the display screen of the analyzer, from being displayed at a deviated position at the lower end of the display screen by the readout pulse being outputted at a faster timing than the predetermined timing and the effective pixel being read out at a premature timing, as well as to prevent the OPB from being displayed at a deviated position at the upper end of the display screen by the readout pulse being outputted later than a predetermined timing and the effective pixel being read out at a retarded timing.
Thus it has not been possible with the above-described solid-state imaging device to shorten the shutter speed or charge storage time to less than the time period beginning at the decay of the vertical synchronizing signal and ending with the outputting of the readout-pulse. Consequently, it has not been possible with the conventional solid-state imaging device to perform imaging with a high-speed shutter operation of e.g. one millionth of a second.